Does {local $SIG{PIPE} = sub {}; close($pipe)} work? My guess is that perl is checking the status of the pipe and emulating SIGPIPE when you close it.

Does it work 'where'? I had it bracketing the write call and added bracketing for the close statement:

    sub close_child_io () {
      Debug "SigCHLD: close iopair[1]";
      local * pipe_catch = sub ($) { Debug "Child already closed";
        $iopair[1] = $iopair[0] = undef; };
      my $tmp = $PathTree::iopair[1];
      $PathTree::iopair[1] = undef;
      $SIG{PIPE}=\&pipe_catch;
      close($tmp);
      $SIG{PIPE}='DEFAULT';
    };
[download]

Only the "using-TNT-to-kill-a-fly" idea of redirecting STDERR to /dev/null before every write through perl seems to shut it up:

|2588     local * write_child = sub ($) {
|2589       my $out=$_[0];
|2590       return unless $iopair[1] and ref $iopair[1] and -w $iopair
+[1];
|2591
|2592       local * pipe_catch = sub ($) { Debug "Child closed";
|2593         $iopair[1] = $iopair[0] = undef; };
|2594
|2595       $SIG{PIPE}=\&pipe_catch;
|2596       no warnings;
|2597       open (OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!
+";
|2598       open (STDERR, ">", "/dev/null") or die "Can't redirect STD
+ERR to /dev/null: $!";
|2599       select STDERR; $|=1;
|2600       P $iopair[1], $out."\n" ;
|2601       open(STDERR, ">&OLDERR") or die "Can't restore stderr: $!"
+;
|2602       $SIG{PIPE}='DEFAULT';
|2603     };
[download]

That sounds like perl is remembering an error from an earlier failed write and reporting it when the pipe is closed or something else entirely is going on...

I finally searched perldiag for "unable to close filehandle" and that message indicates that the "implicit close done by an open got an error indication" so now I must ask: what is sub P?